PGD2 belongs to the class of prostaglandins derived from arachidonic acid. It is the predominant prostanoid produced by activated mast cells and is involved in the pathogenesis of allergic diseases such as asthma, rhinitis and atopic dermatitis (see Lewis et al. J. Immunol. 129:1627 (1982), Hardy et al., N. Eng. J. Med. 311: 209 (1984), Murray et al., N. Eng. J. Med. 315: 800 (1986), Barry et al., Br. J. Pharmacol. 94:773 (1988). PGD2 is a ligand for the DP receptor and was initially thought to elicit all its biological actions through this receptor. The role of the DP receptor in allergic asthma has been demonstrated with DP deficient mice (see Matsuoka et al Science 287: 2013 (2000)). More recently PGD2 was identified as the ligand for another G-protein coupled receptor referred to as “chemoattractant receptor-homologous molecule expressed on Th2” or simply “CRTH2” (see Tanaka et al., J. Immunol. 164:2277 (2000), and U.S. Patent Application Publication No. US2002/0022218). CRTh2 is expressed on basophils, eosinophils and immune helper cells of the Th2 type. Th2 cells have been shown to be involved in the orchestration of allergic response (see Wills-Karp, Annual Review of Immunology, 17: 255 (1999)). It has been shown that PGD2 induces chemotaxis in Th2 cells and eosinophils via the CRTH2 receptor, suggesting that CRTh2 may play a pro-inflammatory role in allergic diseases (see Hirai et al. J. Exp. Med. 193: 255 (2001). It has also been shown that in atopic
dermatitis patients there is an increase in circulating T cells expressing CRTh2 which correlates with the severity of the disease. (see Cosmi et al. Eur. J. Immunol. 30: 2972(2000), Iwazaki et al. J. Investigative Dermatology, 119: 609 (2002). Thus, PGD2 is involved in various aspects of inflammation through its receptors DP and CRTh2. Antagonists of CRTH2 and DP are therefore expected to be useful in the treatment of PGD2 mediated disorders. Unfortunately, there are few if any known CRTH2 inhibitors. As a consequence, clinicians will be unable to exploit these discoveries until new CRTH2 inhibitors are developed.